What if everything you knew about your cause was a lie? What if you found out you were fighting for the wrong side? What if your world came crashing down around you and up was down, left was right, evil was good, and good was evil?
This is what happens to Harry when he runs away from the Burrow after Dumbledore unceremoniously drops him there before sixth year. As it was Severus Snape's turn to watch Harry that summer, he finds the boy badly injured and takes him back to the Prince manor to properly heal him. He calls for Voldemort using his mark, and explains what happened. That Potter may very well be 'turnable' and believe their story, even though he sees Dumbledore as a hero.
When Harry wakes up, Voldemort is standing over him holding his wand and performing a medical scan. Harry discovers that Voldemort is leading the real light side while Albus Dumbledore has been manipulating everyone all along to form the world he wanted, going as far as defeating Grindelwald and manipulating people into believing that he was fighting for equality. Going as far as making the world think that Tom Riddle was responsible for the slew of muggle and muggle-born murders because Tom knew what he was up to, so he led an uprising. Albus was the one who came up with the name Voldemort; he wanted to give Tom a dangerous-sounding name to go along with the story he developed. Tom took it on gladly. "You want me to be the bad guy? Fine. I'll be the bad guy."
But it was Albus Dumbledore who killed Harry's parents. Lily and James were on Tom's side. They were his loyal followers. To be fair, James was only there for Lily, but they still knew the real truth. Not the truth Dumbledore fed to the world. Unfortunately for them, Albus discovered Harry's prophecy via his connections at the Ministry. Tom found out, and both of them thought that the Dark Lord the boy was destined to kill was Voldemort. Lily begged for Voldemort to spare them, spare their unborn child, to which he agreed - he would never kill a baby, nor anyone who had done nothing wrong. He told them to hide and make it seem like they were hiding from him. He told them to elect a Secret Keeper they could trust, someone who would vow to keep the information from Dumbledore as well as himself. Who better than Sirius Black?
And so it was done. However, none of them could have guessed that the mousey Peter Pettigrew was a double agent working for Dumbledore. He convinced the couple to change their Secret Keeper to him because Albus would never suspect him - to which they hesitantly agreed. Then he gave up their location to the Hogwarts professor, along with the next time the Dark Lord was visiting the Potter home. Dumbledore murdered them and framed Voldemort, describing the prophecy to anyone who was important enough to know. They believed him and civilians did not need an explanation, for this so-called Dark Lord had been terrorizing them for years. According to Dumbledore's well-placed manipulations.
The carnage happened just before Voldemort apparated to their house. He walked into a war zone. Scrambling to find anyone he could save, he only found Harry crying in his crib. Distraught, he moved to pick Harry up and leave but then Dumbledore stepped out from the shadows, wand raised at the boy. Without a second thought, Voldemort attacked. After a flurry of curses cast by both, the older man managed to get to Harry. He held Harry up as a shield just as Voldemort cast the killing curse, but because of Lily's love magic, it rebounded and destroyed Voldemort's body, leaving baby Harry with a lightning-shaped scar. The rest, as they say, is history.
Until now.
Long before and long after Voldemort regained his body in Harry's fourth year, he thought the Dark Lord mentioned in Harry's prophecy was himself and even though he told Lily all those years ago that he would protect her son, his cause was more important. Killing Dumbledore and turning the world right again was more important than one boy. So he vowed to do what he thought was necessary to save the world.
Until now.
After telling Harry all of this the next day after he came to Prince Manor, he realized that the Dark Lord in the prophecy wasn't him at all - it was Albus Dumbledore.
Harry spends the remaining summer months at Prince Manor, befriending Draco Malfoy (and discovering his feelings for him) and meeting most of the Death Eaters - people who, like his own parents, remained loyal to the true cause Voldemort began over thirty years ago. Fred, George, Charlie, Bill, Fleur, Luna, Xenophilius Lovegood, Pansy Parkinson, Blaise Zabini, and even Neville and his grandmother, to name a few.
Now, Harry must go back to Hogwarts to carefully recruit as many people as he can and train them, all while pretending he didn't know any of this so Dumbledore will continue to trust him.
